In order to promote adhesion between rubber and ferrous metals it is known to employ a variety of metallic salts or complexes or other additives as coatings to the metal or as an ingredient in a rubber composition.
Representative of the art which discloses the use of an additive ingredient in the rubber stock are three patents which are commonly owned by the Assignee of record herein. U.S. Pat. No. 4,203,874 is directed toward a rubber composition and tire having improved rubber-to-metal adhesion and a method for improving the adhesion therebetween. All three inventions are based on the addition of a transition metal salt of para-aminobenzoic acid with transition metals such as Ni, Cr, Mn, Zn, Mo, Cd, Zn, Ag and Ti.
U.S. Pat. Nos. 4,239,663 is similar to 4,203,874 wherein para-aminobenzoic acid or the cobalt salt thereof is added to improve rubber-to-metal adhesion.
U.S. Pat. No. 4,324,281 also provides for improved rubber-to-metal adhesion in rubber stocks and tires as well as a method for improving the adhesion. The ingredient added for practice of these inventions is a tetracarboxylic dianhydride, one preferred example being 3,3'-4,4'-benzophenone tetracarboxylic dianhydride.
A fourth U.S. Pat. No. 3,993,847, employs a separate adhesive layer and an ingredient for the rubber compound in a process for laminating rubber to a ferrous metal and the laminated articles resulting therefrom. Improved rubber-to-metal adhesion is obtained by using an adhesive layer comprising an interpolymer of a conjugated diene and a polymerizable heterocyclic nitrogen base and containing silica filler. Adhesion is optionally improved further by the addition of a cobalt compound, e.g., carboxylate, chloride or nitrate. A primary objective of the invention disclosed is that the use of the adhesive layer eliminates the need to employ plated steel.
Thus, while others have sought to enhance adhesion between rubber compositions and metals by employing separate adhesive layers and/or various metal salts, complexes or other ingredients in the stock, the art of which we are aware has not disclosed the exclusive use of the compounds set forth hereinbelow in a vulcanizable rubber stock to increase adhesion properties between rubber and metallic reinforcement, particularly brass-plated steel.